Method for improved mud pulse telemetry

ABSTRACT

The present invention is a method for improved mud pulse telemetry. Mud pulse signals generated by a downhole mud pulser are detected at the surface by monitoring mud flow rate. Mud flow rate can be monitored by use of a flow meter placed downstream from the surge suppressor.

FIELD OF THE INVENTION

The present invention relates to measurement while drilling operations.More particularly, the present invention relates to a method forimproving the performance of measurement while drilling operations whichutilize mud pulse telemetry.

BACKGROUND OF THE INVENTION

In petroleum and related wellbore drilling operations, there has longbeen a need for a measurement while drilling (MWD) method capable oftransmitting real time data from inside a wellbore during drilling. Realtime data concerning the drill bit and the formation being penetratedcan be of great value to drilling personnel in making the best use ofmanpower and equipment.

There are at least four basic data telemetry methods which are currentlybeing developed for MWD operations. These methods include the telemetryof data by electromagnetic radiation transmitted through the earth, byelectric current transmitted through insulated conductor wires, byacoustical pulses transmitted through a drill string, and by pressurepulses transmitted through drilling mud. To date, only the lastmentioned method, commonly known as mud pulse telemetry, has foundcommercial success.

In the drilling of oil and gas wells, drilling mud is typicallycirculated down the interior of a hollow drill string, through nozzlesin a drill bit located at the bottom of the drill string, and back up tothe surface through the annulus between the drill string and the wall ofthe wellbore. Large pumps, generally of the reciprocating variety, areused to circulate the drilling mud. A surge suppressor is typicallylocated on the mud flow line between the mud pump and the drill stringto smooth the flow coming from the pump. The primary functions of thedrilling mud are to lubricate the drill bit, to transport rock cuttingsto the surface, and to maintain a hydrostatic pressure in the wellboresufficient to prevent the intrusion of formation fluids and therebyprevent blowouts.

Mud pulse telemetry utilizes the column of drilling mud which extendsthrough the interior of the drill string or annulus as a telemetry linkbetween downhole instruments and surface receiving equipment. Thedownhole insruments are generally contained in a drill string instrumentsub located near the bottom of the drill string. These instruments areusually linked to a mud pulser contained in another drill string subpositioned adjacent to the instrument sub. The mud pulser generatespressure pulses in the drilling mud in response to signals received fromthe instruments. These pressure pulses are typically generated in themud pulser by alternately opening and closing valves or vents throughwhich the drilling mud flows. Closing and opening the valvesrespectively increases and decreases backpressure on the drilling mud.Each change in pressure constitutes a mud pulse signal, and the mudpulse signals typically form a binary code which carries the soughtafter information. These mud pulse signals are detected by a pressuretransducer located at the surface. The pressure readings from thepressure transducer are processed and interpreted to decode the mudpulse signals and thereby yield information concerning downholeconditions.

At least two major technical problems have confronted mud pulsetelemetry. The first problem concerns data transmission rates. Mudpulsers can be designed to generate mud pulse signals at frequenciesexceeding one pulse per second. However, resolving such rapid mud pulsesignals from one another at the surface has to date proved impractical.Hence, mud pulse signal frequencies of less than about one pulse everyfive seconds have generally been employed. If possible, it would behighly advantageous to increase mud pulse signal frequency, therebyincreasing data transmission rates. Much effort has gone into thedevelopment of electronic data processing systems to improve mud pulsesignal detection and decoding, but few have succeeded in increasing datatransmission rates much beyond one mud pulse signal every five secondsor so.

The second major technical problem confronting mud pulse telemetry isthat the pressure pulses generated by the mud pulser can be difficult toextract from pressure variations caused by other sources. Pressurevariations caused by other sources constitute noise which tends toobscure the mud pulse signals. This noise is primarily a consequence ofthe moving pistons, valves and other mechanical components that make upthe mud pump. Overcoming this noise problem has required the developmentof mud pulsers which generate powerful pressure pulses, and also thedevelopment of sophisticated electronic data processing systems.

There still exists a great need for a mud pulse telemetry method whichovercomes the above-mentioned problems. The present invention is aimedat providing such a method.

SUMMARY OF THE INVENTION

The present invention overcomes the above-mentioned problems bymonitoring changes in mud flow rate caused by a mud pulser. We havediscovered that monitoring mud flow rate rather than mud pressure canresult in faster data transmission rates due to improved resolution ofmud pulse signals from one another. At the surface, mud flow rateresponds more sharply to a downhole mud pulser than does mud pressure.In addition, signal to noise ratios for mud flow rate are much higherthan for mud pressure. As a result, mud pulsers used in practicing themethod of the present invention can be less powerful, more energyefficient and more reliable than those required for practicing prior artmethods. Another benefit of the present invention is that the need forsophisticated data processing equipment to extract the mud pulse signalsfrom background noise is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, partly in section, of a drilling rig whichemploys the mud pulse telemetry method of the present invention.

FIG. 2 is a graphical representation of mud pressure measurements madeat the surface during operation of a downhole mud pulser.

FIG. 3 is a graphical representation of mud flow rate measurements madeat the surface during operation of a downhole mud pulser.

FIG. 4 shows four hypothetical graphs which serve to compare the time ittakes for surface pressure measurements and surface flow ratemeasurements to respond to mud pulse signals generated by a downhole mudpulser.

FIG. 5 is a graphical representation indicating the signal to noiseratio for mud pressure measurements made at the surface.

FIG. 6 is a graphical representation indicating the signal to noiseratio for mud flow rate measurements made at the surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a side view, partly in section, of a drilling rigwhich employs the mud pulse telemetry method of the present inventioncan be seen. Wellbore 10 has been drilled into the earth to recoverpetroleum or other valuable resources. Drill string 11 is rotated byrotary table 12 which causes drill bit 13 to penetrate subsurfaceformation 14. Drilling mud is circulated by mud pump 15 downward throughthe hollow interior of drill string 11, through nozzles (not shown) indrill bit 13, and back up to the surface through annulus 16 betweendrill string 11 and the wall of wellbore 10. Drilling mud returning tothe surface from annulus 16 flows through mud return line 17 into mudpit 18. Shale shaker 19 may be used to remove formation cuttings fromthe drilling mud as it returns to the surface.

Mud pump 15 draws drilling mud 20 from mud pit 18 and pumps the drillingmud through mud flow line 21, rotary hose 23, swivel connection 24,kelly 25 and drill string 11. Surge suppressor 26 is located on mud flowline 21 near the outlet of mud pump 15 to smooth out flow and pressuresurges caused by the mud pump. Surge suppressers are well known to thoseskilled in the art.

Downhole instruments (not shown) are located inside instrument sub 27,which is made up on drill string 11 near drill bit 13. As is well known,various types of instruments can be contained in the instrument sub,including instruments for measuring formation pressure, temperature andconductivity and for measuring drill bit orientation and wear. Theseinstruments generate signals, typically electrical, which arerepresentative of the downhole information being collected. The signalsare relayed to mud pulser sub 28 which is made up on drill string 11adjacent to instrument sub 27. The mud pulser sub contains a mud pulser(not shown) which has valves, vents or other means for restricting theflow of drilling mud through the drill string in response to the signalsfrom the instrument sub. For the sake of simplicity, the operation ofmud pulsers which restrict flow by the opening and closing of one ormore valves will be described. These and other types of mud pulsers arewell known to those skilled in the art. Each time the mud pulser valvesare opened or closed, a mud pulse signal is generated which propogatesupward to the surface through the mud inside the drill string. The mudpulse signal comprises a change in the rate of mud flow, which isaccompanied by a corresponding change in pressure.

Mud pulse signals are detected at the surface by flow meter 29, which ispositioned on mud flow line 21 downstream from surge suppressor 26.Suitable flow meters for use in practicing the method of the presentinvention are magnetic flow meters such as the Foxboro Series 2800magnetic flow meters manufactured by The Foxboro Company of Foxboro,Mass. Other types of commercially available flow meters, such asinsertion type flow meters, can also be employed. Magnetic flow metersoperate by establishing a magnetic field through which the slightlyconductive drilling mud flows, thereby creating an electric potential.This potential, which is proportional to the rate of flow, is measuredand amplified by electronics (not shown) associated with the magneticflow meter. For a Foxboro Series 2800 magnetic flow meter, a FoxboroSeries E96R transmitter can be used. These amplified measurements aresent by the transmitter to a strip chart recorder (not shown) and/ordata processor 30, which processes and communicates the downholeinformation to drilling personnel. Suitable strip chart recorders anddata processors are well known to those skilled in the art. Bymonitoring flow rate rather than pressure in accordance with the methodof the present invention, mud pulse detection at the surface is greatlyenhanced.

Referring to FIG. 2, a graphical representation of mud pressuremeasurements made at the surface during operation of a downhole mudpulser can be seen. The graph is typical of prior art methods ofdetecting mud pulse signals and provides a comparison to the method ofthe present invention. In a test well approximately 1000 feet (305meters) deep, a mud pulser contained in a mud pulser sub near the bottomof the drill string was driven by a clock circuit in order to transmitmud pulse signals in a square wave pattern with a ten second cycle.Thus, the mud pulser valves were alternately opening and closing onceevery five seconds. In essence, the clock circuit replaced theinstrument sub which would be used in an actual MWD operation. The drillstring was not rotated for the test. The test well setup was similar tothat shown in FIG. 1, except that a strain gauge type pressuretransducer was placed in the mud flow line near the flow meter toprovide the desired comparison. Mud pressure was recorded everyone-fourth of a second.

The plot of pressure versus time shown in FIG. 2 exhibits a sawtoothpattern rather than a square wave pattern. A square wave pattern wouldbe expected if mud pulse resolution were precise. Thus, FIG. 2 shows theimprecise resolution typical of prior art methods which rely on pressuremeasurements at the surface. Although the individual pressure pulses canbe discerned from the sawtooth pattern, a substantial increase in pulsefrequency would result in signal loss due to insufficient resolution.The large change in pressure seen at about 125 seconds resulted from adecrease in mud pump speed, which caused a reduction in overallpressure.

FIG. 3 shows the readout from a Foxboro 3-inch Series 2800 magnetic flowmeter in accordance with the method of the present invention. The flowmeter was positioned on the mud flow line downstream from the surgesuppressor. Mud flow rate was recorded every one-fourth of a second. Theflow meter recording was made at the same time as the pressuretransducer recording shown in FIG. 2. Thus, the time scales on FIGS. 2and 3 are contemporaneous. As can be clearly seen by referring to FIG.3, the recording of flow rate approaches a square wave much more closelythan the recording of pressure shown in FIG. 2. The difference can beattributed to improved resolution of the mud pulse signals using themethod of the present invention. The improved resolution apparentlyresults from faster response of flow rate than pressure to the mud pulsesignals being generated by the downhole mud pulser. With the improvedresolution achieved by the method of the present invention, thefrequency of the mud pulser could be substantially increased to a levelthat is not practical for prior art methods. In this fashion, fasterdata transmission ratees can be achieved using the method of the presentinvention, with less need for sophisticated data processing equipment todetect and decode the signals being transmitted.

The discovery that flow rate responds more quickly than pressure to thesignals being generated by the mud pulser at first seemed to present aparadox. As is well known, restricting a passageway through which afluid is flowing decreases the rate of flow and at the same timeincreases backpressure on the fluid upstream from the restriction.Assuming constant power output from the mud pump, the relationshipbetween pressure and flow will be an inverse linear one. This is evidentfrom the following well known equation which gives the relationship ofpump power to pressure and flow: Hydraulic Horsepower equals Pressure(psi) times Flow Rate (gallons per minute) divided by 1714. Thus, onewould expect flow rate to respond to a restriction caused by a downholemud pulser no more quickly and with no more relative amplitude thanpressure. The pressure and flow rate changes associated with the mudpulse signal should propogate together to the surface. As a result, onewould expect to gain no advantage by monitoring flow rate instead ofpressure to detect mud pulse signals. Yet, as can be seen by comparingFIGS. 2 and 3, signal resolution is greatly enhanced when flow rate ismonitored in accordance with the method of the present invention.

The key to explaining this apparent paradox seems to lie in the surgesuppressor. As mentioned above, surge suppressors are typicallyinstalled on the mud flow line between the mud pump and the drill stringto smooth out variations in flow caused by the pump. Surge suppressorsare usually charged with a pressurized gas. This gas acts as a damper tosmooth out fluctuations in flow and pressure. For example, if flow fromthe mud pump suddenly increases, the gas in the surge suppressor iscompressed by the inflowing fluid, thus creating room for the excessfluid to be diverted into the surge suppressor and temporarily storedinside. As flow from the mud pump returns to normal, the gas in thesurge suppressor expands to force the excess fluid out of surgesuppressor and into the mud flow line. In this manner, the surgesuppressor smooths out fluctuations in the mud flow and pressure causedby the mud pump. The action of the surge suppressor is desirable fromthe standpoint of maintaining a steady flow of drilling mud into thewell at a constant pressure, but is undesirable from the standpoint oftrying to measure mud pressure changes generated by a downhole mudpulser. The surge suppressor is designed to function as a pressuredamping device and hence attempts to attentuate all transient pressurechanges, including those generated by a mud pulser.

Consider the following. If a steady state flow of drilling mud issuddenly restricted by the action of a mud pulser, pressure in the mudflow line increases and thereby exceeds the pressure of the gas in thesurge suppressor. As a result, mud is forced into the surge suppressor.As the surge suppressor begins to fill with drilling mud, thepressurized gas compresses and its pressure increases. When the gas hascompressed a certain amount, the pressure in the surge suppressor andthe pressure in the mud flow line will be balanced. When the balance isreached, flow to and from the surge suppressor is reduced to zero. Ifthe restriction in the mud pulser is then opened to generate another mudpulse signal, the pressure in the mud flow line decreases. As a result,the pressure in the mud flow line becomes less than the pressure of thegas in the surge suppressor. Consequently, the gas in the surgesuppressor expands and flushes out the excess drilling mud which filledthe surge suppressor while the mud pulser was restricting flow. The flowof drilling mud from the surge suppressor continues until the pressuresare balanced and flow to and from the surge suppressor is again zero.

The time it takes to go from one steady state to another is the time ittakes to detect the full amplitude of the mud pulse signal generated bythe mud pulser. If not for the surge suppressor, this time should beequal for flow rate measurements and pressure measurements. However,apparently due to the damping action of the surge suppressor, the timeinterval between steady states is much greater for pressure measurementsthan for flow rate measurements. Thus, the pressure measurementsutilized by prior art MWD methods have a slower response time than theflow rate measurements utilized by the method of the present invention.The apparent reason for the difference in response times will now beexplained in greater detail with reference to FIG. 4.

FIG. 4 consists of four hypothetical graphs showing mud pressure and mudflow rate changes caused by a mud pulser. All four graphs arecontemporaneous, as indicated by the common time scale. Graph 4A shows aplot of pressure in the mud flow line downstream from the surgesuppressor versus time. Graph 4B shows a plot of the rate of mud flowfrom the mud pump versus time. Graph 4C displays the rate of mud flowfrom the surge suppressor versus time. Graph 4D shows the rate of mudflow into the well versus time, as measured in the mud flow linedownstream from the surge suppressor. The rate of mud flow into the wellshown in Graph 4D is the rate of mud flow from the surge suppressorshown in Graph 4C plus the rate of mud flow from the mud pump shown inGraph 4B.

Prior to time t₁, a steady state condition exists with the valves in themud pulser being in a closed position. Mud flow line pressure is at P₁(Graph 4A) and the rate of mud flow from the mud pump is at Q₁ (Graph4B). The rate of mud flow into the well is also at Q₁ (Graph 4D) becausethere is no additional flow from the surge suppressor (Graph 4C), asexpected during steady state conditions.

At time t₁, the mud pulser valves are opened to transmit a signal to thesurface. Mud flow line pressure (Graph 4A) steadily drops in response tothe decrease in backpressure caused by the opening of the valves. At thesame time, the rate of mud flow from the mud pump (Graph 4B) steadilyincreases as a result of the drop in backpressure. The inverse linearrelationship of Graphs 4A and 4B indicates that the mud pump is workingat a constant power output. At time t₃, a new steady state is reachedwith mud flow line pressure at P₂ (Graph 4A). The interval between t₁and t₃ on Graph 4A is the response time for detection of the fullamplitude of the mud pulse signal generated by the mud pulsar usingprior art methods.

Graph 4D shows the greatly reduced response time which results from useof the method of the present invention. Measurement of the rate of mudflow into the well shows that new steady state rate of flow Q₂ isreached at time t₂. The response time is the time interval between t₁and t₂, which is much shorter than the interval between t₁ and t₃. Thegreatly reduced response time of mud flow rate compared to mud pressureas measured at the surface is apparently attributable to the flow fromthe surge suppressor (Graph 4C).

When the mud pulser valves are opened at time t₁ to transmit a mud pulsesignal, the resulting decrease in backpressure causes the surgesuppressor to expel the excess drilling mud which collected in it whilethe valves were closed, as explained above. This flow rate from thesurge suppressor (Graph 4C) combines with the flow rate from the mudpump (Graph 4B) to yield the flow rate into the well (Graph 4D). Theflow rate from the surge suppressor reaches a maximum at about t₂ andthen gradually decreases to zero as the flow rate from the mud pumpgradually increases to new steady state Q₂ at time t₃. The result isthat the new steady state flow rate Q₂ into the well (Graph 4D) isreached at time t₂ long before it is reached in the mud pump at time t₃.

With the reduced response time achieved by monitoring the rate of mudflow in accordance with the method of the present invention, it isexpected that data transmission rates can be greatly increased overthose which are practical using pressure measurements as taught by theprior art. However, as mentioned above, this is not the only advantagethat the method of the present invention has over prior art methods. Inaddition, signal to noise ratios are improved. As a result, the mudpulse signals can be extracted much more readily from background noise,which is caused primarily by the mud pump. Consequently, the need forsophisticated data processing equipment and powerful mud pulsers isreduced, thereby yielding cost savings. A further advantage is that theimproved signal to noise ratio may make it practical to receive mudpulse signals from a mud pulser during periods of low mud flow ratesoften associated with well control problems. Real time downholeinformation is especially valuable during such periods and canpotentially aid in the prevention of blowouts. Signal to noise ratiosfor prior art methods which monitor pressure changes are generally toolow during periods of low mud flow rates to permit adequate detection ofthe signals. Thus, with prior art methods, the mud pulse signals can beunavailable when they are most needed.

FIGS. 5 and 6 permit a signal to noise ratio comparison between themethod of the present invention and the prior art methods which relysolely on pressure measurements. The mud pressure and flow raterecordings respectively shown in FIGS. 5 and 6 were generated with thesame test well setup described above with reference to FIGS. 2 and 3.FIG. 5 shows a recording from a strain gauge type pressure transducerlocated in the mud flow line downstream from the surge suppressor. Mudpressure was recorded every one-fourth of a second. The recording wasmade with the valves of the mud pulser open at all times. Thus, no mudpulse signals were being generated. The relatively large pressureincreases seen at about 50, 125, 225, 300, 400 and 500 seconds werecaused by increases in mud pump speed.

In the absence of noise, one would expect to see a smooth horizontalline corresponding to each of the different mud pump speeds since no mudpulse signals were generated. However, due to pressure variations causedprimarily by the mud pump, the lines are not smooth but instead showsubstantial fluctuations in pressure. These fluctuations constitutenoise which tends to obscure mud pulse signals in the prior art methods.The erratic and largely nonperiodic trace seen in FIG. 5 between 0 andabout 300 seconds resulted because the pressure of the gas inthe surgesuppressor (about 900 psi) greatly exceeded the mud pressure at thelowest mud pump speeds. Hence, the surge suppressor was unable toeffectively dampen pressure surges from the mud pump. Thus, it can beseen that it would be especially difficult to detect mud pulse signalsusing pressure measurements at low mud pump speeds. In general, thelower the mud pump speed, the lower the signal to noise ratio and thepoorer the signal detection.

FIG. 6 shows a recording of flow rates taken from a Foxboro 3-inchSeries 2800 magnetic flow meter positioned on the mud flow linedownstream from the surge suppressor and near the pressure transducerwhich was used to generate FIG. 5. Mud flow rate was recorded everyone-fourth of a second. The time scale in FIG. 6 is contemporaneous withthat in FIG. 5. Thus, the recordings shown in FIGS. 5 and 6 were madesimultaneously. As can be readily seen in FIG. 6, the horizontal flowrate lines corresponding to the different mud pump speeds are muchsmoother than the pressure measurements shown in FIG. 5. This indicatesthat much less noise is being measured by the flow meter. The decreasednoise makes it much easier to see the changes in mud pump speed at about50, 125 and 225 seconds in FIG. 6 than in FIG. 5. Likewise, thedecreased noise would make it much easier to detect mud pulse signals,which generally create flow rate and pressure changes of far lessmagnitude than those accompanying increases and decreases in mud pumpspeed.

The low noise associated with the method of the present invention forthe most part can be attributed to the action of the surge suppressor,which functions to smooth out variations in mud pump flow rate, asexplained above. Perhaps just as significant as the low overall noiselevel is the observation from FIG. 6 that the level of noise isrelatively independent of pump speed when compared to prior art methods.This phenomenon may permit the method of the present invention toresolve mud pulse signals at the low mud flow rates often associatedwith well control problems.

The reduced noise which is characteristic of the method of the presentinvention creates other benefits as well. Due to improved signal tonoise ratios, less powerful mud pulsers can be used. Such mud pulsershave lower energy requirements, are less expensive to build, and aremore durable. In addition, the need for sophisticated data processingequipment to extract the desired mud pulse signals from the backgroundnoise is greatly reduced. Therefore, the method of the present inventionshould enhance the performance of measurement while drilling operationswhich utilize mud pulse telemetry.

In addition to decreased background noise, there is another reason whythe method of the present invention enhances mud pulse signal detectionat low mud flow rates. As mud pump speed decreases, flow rate andpressure drop. As a result, the absolute magnitude of the changes inpressure and flow rate generated by a mud pulser also drop. However,they do not drop by the same factor. The magnitude of the pressurechanges generated by the mud pulser will drop to a greater extent thanthe magnitude of the flow rate changes. This difference is a consequenceof the well known laws of fluid dynamics.

Inasmuch as the present invention is subject to many variations,modifications and changes in detail, it is intended that all subjectmatter discussed above and shown in the accompanying drawings beinterpreted as illustrative and not in a limiting sense. For example,different flow meter placements can be used. Other variations,modifications and changes in detail will be obvious to those skilled inthe art. Such variations, modifications and changes in detail areincluded within the scope of this invention as defined by the followingclaims.

We claim:
 1. A method of obtaining information from a wellbore which isbeing drilled, said wellbore containing a drill string through whichdrilling mud is flowing, said method comprising the steps of:(a) makingmeasurements of one or more downhole parameters with one or moreinstruments positioned proximate to the lower portions of said drillstring; (b) generating changes in the flow rate of said drilling mud inresponse to and indicative of said measurements; and (c) monitoring theflow rate of said drilling mud proximate to the surface to detect saidchanges and thereby obtain said information.
 2. The method of claim 1wherein step (c) is the primary means for detecting said changes.
 3. Themethod of claim 1 wherein step (c) is the sole means for detecting saidchanges.
 4. The method of claim 1 wherein drilling mud pressure is notmonitored to obtain said information.
 5. The method of claim 1 whereinsaid changes in the flow rate of said drilling mud are generated by amud pulser positioned in said drill string, said changes in flow rateconstituting mud pulse signals.
 6. The method of claim 5 wherein a flowmeter is used to monitor said flow rate and thereby detect said mudpulse signals.
 7. The method of claim 6 wherein said flow meter is amagnetic flow meter.
 8. The method of claim 6 wherein a mud pump is usedto flow said drilling mud through a mud flow line and into said drillstring, said mud flow line being in fluid communication with said mudpump and said drill string, wherein a surge suppressor is positioned onsaid mud flow line between said mud pump and said drill string, andwherein said flow meter is positioned to monitor said flow rate in saidmud flow line between said surge suppressor and said drilling string. 9.The method of claim 8 wherein the mud pulse signals detected by saidflow meter are processed to obtain said information.
 10. An improvedmethod for obtaining information from a wellbore as it is being drilled,said wellbore containing a drill string through which drilling mud isflowing, said drilling mud being pumped into said drilling through a mudflow line, said drill string having a plurality of instrumentspositioned proximate to the lower portions thereof, said instrumentsbeing capable of making measurements of downhole conditions and ofcreating signals indicative of said measurements, said drill stringfurther having a mud pulser which is adapted to receive said signalsfrom said instruments and to generate mud pulse signals indicative ofsaid measurements, wherein the improvement comprises measuring the flowrate of said drilling mud proximate to the surface to receive said mudpulse signals.
 11. The method of claim 10 wherein the pressure of saiddrilling mud is not measured to receive said mud pulse signals.
 12. Themethod of claim 11 wherein a surge suppressor is deployed on said mudflow line and wherein the flow rate through said mud flow line ismeasured between said surge suppressor and said drill string.
 13. Themethod of claim 12 wherein a flow meter is used to measure said flowrate.
 14. The method of claim 13 wherein said flow meter is adapted toprovide an output representative of the measured flow rate and whereinsaid output is processed to detect and decode said mud pulse signals andthereby obtain said information.